The present invention relates to a means for separately controlling the electromagnetic radiation, and specifically radio frequency (RF) radiation, in the horizontal direction and at angles of elevation above the horizon from an antenna by means of both horizontally and vertically oriented wires or other conductors each of which is properly energized.
The present invention further provides means for combining groups of wires or conductors arranged vertically, horizontally or at slant angles so as to generate radiation patterns in the horizontal direction to produce a ground wave signal that appropriately serves certain areas while separately controlling the antenna radiation pattern at angles above the horizontal (at elevations) so as to minimize the sky wave signal and thus limit the sky wave interference to the service areas of other stations.
In the medium wave AM radio band (300-3000 kHz) directional antennas are frequently used both to provide service to specified areas and to minimize interference with other radio stations. There are two basic modes of radio wave propagation which occur at these frequencies. During both the daytime and nighttime a ground wave signal is propagated. This ground wave signal is normally the desired received signal. Further, during the nighttime, a second method of propagation occurs; a signal is reflected from the ionosphere and returns to earth at significant distances from the transmitting station with substantially less attentuation than that suffered by the ground wave, thus causing more interference to other stations during the nighttime. Conventional directional antennas suppress both the ground wave and sky wave signals and consequently the service area may be reduced and some desired service may be lost in order to minimize sky wave interference to other stations.